Ink-jet recording is a recording method to record images and letters with ejection of minute ink droplets by various working principles onto a recording sheet, such as paper, exhibiting advantages of relatively high speed, low noise and simplified multi-color printing.
Nozzle clogging and maintenance of the foregoing recording method has been improved in both ink and apparatus, and in recent years, ink-jet recording has rapidly become popular in various fields of printers, facsimile machines and printers for computers.
Recently, ink-jet printers have specifically resulted in enhancement of image quality which is approaching that of conventional silver halide photography, and thus, it is required that a recording sheet also duplicates conventional photographic quality, and then reproduces the aesthetic property of silver halide photography (such as glossiness, smoothness and stiffness).
As one of methods to reproduce said aesthetic property of silver halide photography, a so-called swelling type recording sheet has become common, comprised of a hydrophilic binder, such as gelatin and polyvinyl alcohol, applied onto a support. However, this method has weaknesses of low ink absorption rate, a tendency of a sticky surface after printing, and image bleeding due to ambient humidity during storage. Specifically, ink droplets are easily blended before they are absorbed due to low ink absorption rate, resulting in bleeding into different colors and beading or unevenness in the same color. Thus, achievement of the desired silver halide photographic image quality has been rather difficult.
So-called void type recording sheets are becoming the main types instead of the foregoing swelling types. High ink absorption rate is a major feature due to effective ink absorption into minute voids. Examples of the recording sheets which approach silver halide photograph image quality and silver halide photograph aesthetic property are described in JP-A Nos. 10-119423, 10-119424, 10-175364, 10-193776, 10-193776, 10-217601, 11-20300, 11-106694, 11-321079, 11-348410, 10-178126, 11-348409, 2000-27093, 2000-94830, 2000-158807, and 2000-211241 (hereinafter, JP-A refers to Japanese Patent Application Publication).
In addition to image quality and aesthetic property, durability and image stability are highly demanded. Many attempts have been made to achieve the same levels of light stability, moisture resistance and water fastness as those of silver halide photograph. Examples of technology to enhance light stability are disclosed in JP-A Nos. 57-74192, 57-87989, 57-74193, 58-152072, 64-36479, 1-95091, 1-115677, 3-13376, 4-7189, 7-195824, 8-25796, 11-321090, 11-277893, 2000-37951 and others.
Regarding void type recording sheets, they tend to cause problems such as discoloration with specific gases due to its void structure in addition to other light stability problems. Specifically, phthalocyanine water-based dyes employed in common color ink-jet printers tend to cause such discoloration.
The mechanism of this discoloration property has not yet been fully clarified, but it is assumed that the minute porous structure has a large surface area and the inorganic particles used therein have active surfaces, resulting in decomposition of dyes by minute quantities of reactive gases in the ambient air such as ozone, oxidants, SOx and NOx.
Techniques to minimize the discoloration problem are disclosed, for example, in JP-A Nos. 63-252780, 64-11877, 1-108083, 1-216881, 1-218882, 1-258980, 2-188287, 7-237348, 7-266689, and 8-164664. This discoloration problem is a more prominent deteriorating problem in recording sheets exhibiting photograph image quality having an enhanced minute porous structure, and thus, the relevant art for improvement has not been sufficient to obtain the desired effect, calling for fundamental improvement.
One of the countermeasures to the foregoing problem is to use a swelling-type recording sheet, but on the other hand, it is very difficult to improve the critical matter of slow ink absorption rate by the use of this type of sheet.
The discoloration property is overcome to a certain degree with an ink-jet recording method using pigment inks. However, this technology cannot fully eliminate properties such as bronzing of image on the recording sheet to yield a desirable image quality. In addition, a gas insulating method which uses a laminating process for prints and framed printed photos are known. Moreover, a gas insulating layer forming methods are also are disclosed. Examples are disclosed in JP-A Nos. 53-27426, 59-222381, 62-271781, 11-157207, 11-245507, and 2000-71608, in which after printing is carried out on a image recording sheet containing thermoplastic particles on the surface thereof, a gas insulating layer is formed using a heating or pressure treatment.
Further, a recording sheet comprised of a void layer containing fine organic particles and fine inorganic particles is disclosed in JP-A 63-60784. However, these fine organic particles can only be dissolved in special solvents, not normally used in the organic solvents employed in the common ink-jet printer ink, and thus, cannot presently be employed for normal ink-jet recording.
The inventors of the present invention had found that the specific fine organic particles contained in a coated layer on a recording sheet are very effective in preventing discoloration. Specifically, it is effective to provide a high concentration of these fine organic particles in the surface porous layer.
However, as a result of continued study, the above structure has proved to have a tendency of a slow ink absorption rate, resulting in easily generated beading on printed images. In the future, under the expectation that printing speed will be improved, achievement of an early solution is urgently sought.